Heterochromia is an eye condition which is found in humans, cats and dogs. Heterochromia is a difference in eye colour caused by an excess or lack of melanine pigment. Heterochromia is normally inherited but it can also be caused by disease or injury. There are two kinds of Heterochromia : complete heterochromia and partial heterochromia.Complete heterochromia is very rare in humans and it is when both eyes are two different colours. For example one eye may be blue and the other brown. Partial heterochromia is when a part of the eye is a different colour this can also be called sectoral heterochromia.Central heterochromia is whenever the inner of the iris is a different colour to the midperipheral zone of the iris. The true colour of the iris is the outisde ring of it. In humans the colour of eyes are very vast and different. This is because there are 3 true colours which are used to make the colour of an eye. These three colours are brown, grey and yellow. Central heterochromia has been found present in most cases whenever the pigment melanin is low. This is the most common form of heterochromia found in humans even though it is still rare.Complete heterochromia is more frequently observed in other animal species such as dogs like the siberian husky or cats. Complete heterochromia in other species almost always involves one blue eye. In animals melnain is used to give colour of skin, hair and eyes. In eyes if there is low melanin pigment to none they will be blue, if there is a bit of pigment they will be green and if there is a lot of pigment they will be brown. Therefore in animals with complete heterochromia the blue eye is caused by a very little amount of melanin to none.Causes of heterochromiaHeterochromia can be inherited through autosomal dominant fashion from men or women where the gene is passed on. It can also be caused through injury. David Bowie is an example of this as his eye got hit in a fight and now he has heterochromia. It can also be caused by tumours, inflammation through use of eye drops, or disease.